Hitherto, to carry out dicing for cutting a semiconductor wafer having circuit patterns formed thereon into separate chips, a system that uses a radiation-curable adhesive tape has been developed.
A process for producing semiconductor chips from a wafer, for example, comprises the steps of
(1) dicing (a wafer is secured by a strong adhesive force of adhesive tape and, then is cut into separate chips),
(2) irradiation (the film side of the base is irradiated with radiation, to lower the adhesive force of the adhesive tape),
(3) expanding (the adhesive tape is radially expanded, to enlarge the interval between chips), and
(4) picking up (each chip is picked up from the surface of the tape using a collet by vacuum suction and sticking up the tape from the back side of the tape).
Further, there is a process that the expanding (3) is carried out between the steps of dicing (1) and irradiation (2).
In this system, as the adhesive tape, an adhesive tape for securing a semiconductor wafer is used that is made up of a support film through which radiation, such as light, including ultraviolet rays or ionizing radiation, such as an electron beam, can transmit, and an adhesive layer that is coated on the support film, which layer is characteristically curable by exposure to radiation. In this system, the adhesive force of the adhesive tape for securing chips in the dicing step is kept high, and after the semiconductor wafer is cut and separated into chips by the irradiation step, the adhesive force is lowered. Namely, after the cutting and separation into chips, the film side of the support is irradiated with radiation, to cure the radiation-curable adhesive layer, thereby lowering considerably the chip-securing adhesive force, so that, irrespective of the size of the chips, even chips having a size, for example, of 25 mm.sup.2 or over, can easily be picked up. This is because a radiation-curable compound contained in the adhesive layer of the semiconductor wafer-securing adhesive tape, which tape is formed by applying a radiation-curable adhesive on a support film through which radiation can be transmitted, is cured by irradiation with radiation, to cause the adhesive to take on a three-dimensional structure, thereby lowering the fluidity of the adhesive remarkably.
However, due to the three-dimensional structure, the rubber-like elasticity of the adhesive tape possessed at the time of dicing is eventually almost lost at the time of expanding after the curing. Consequently, a problem arises that the conventionally performed enlargement of the intervals between chips by radial expanding of the adhesive tape becomes difficult.
To solve the problem, the use of a soft polyvinyl chloride (PVC) as a central layer in a support film has already been put into practice. However, as polyvinyl chloride resins contain chlorine and further contain stabilizers, including metal compounds, and plasticizers, on some occasions chlorine ions, metal ions, plasticizers, and the like bleed, causing the surface of semiconductor wafers to be contaminated.
Therefore, as disclosed in Japanese Patent Application (OPI) No. 215528/1990, adhesive tapes are suggested, which use a support film comprising a laminated film having a center layer made of a thermoplastic resin with rubber-like elasticity, such as a polybutene-1, a polyurethane, a polyester elastomer, a 1,2-polybutadiene, a hydrogenated styrene/isoprene/styrene copolymer, and a styrene/ethylene/butene/styrene (SEBS) copolymer. However, although all of these adhesive tapes can be enlarged by expanding before the irradiation with radiation, to secure intervals between chips to only prevent contact of the chips, the following problems arise. It is the case that large and uniform intervals between chips are required for a picking-up apparatus involving image recognition after irradiation with radiation. However, none of above prior adhesive tapes satisfies this need sufficiently, because such problems arise as the occurrence of necking of the support film itself (the occurrence of partial extension due to failure of transmission of power at the time of expanding of the film), and the loss of rubber-like elasticity resulting from deterioration of the support film due to the irradiation with radiation.
Considering the above-mentioned polymers individually, a polybutene-1, a polyester elastomer, and a 1,2-polybutadiene are liable to result in necking, thereby making impossible the enlargement of intervals between chips (like PVC); a polyurethane is too high in rubber-like elasticity, thereby leading to a lack of uniformity of intervals between chips; and, although hydrogenated styrene/isoprene/styrene copolymers have preferable properties over the above polymers, they still cannot obviate the problem of necking, and they are attended with the defect that the expansion between chips is unsatisfactory. In Japanese Patent Application (OPI) No. 215528/1990, as a styrene/ethylene/butene/styrene (SEBS) copolymer, RABALON, manufactured by Mitsubishi Petrochemical Co., Ltd., is used in an Example, but Rabalon is blends of SEBS having a molecular weight of 100,000 or more with a polypropylene or ethylene/propylene/diene copolymers, and a compatibilizing agent bleeds into the adhesive layer, thereby making the adhesive force unstable, leading to such problems as the occurrence of picking-up failure and contamination of the wafer. Further, in the case wherein only SEBS is used instead of its blend, in the center layer, depending on the content of styrene and the molecular weight, such problems arise as, for example, necking causes insufficient intervals between chips. There may occur an insufficient lowering of the adhesive force after irradiation due to the poor transmission of radiation.